The present invention relates to the protection of electrical circuits and, more particularly, to the detection of electrical faults of the type known as arcing faults in an electrical circuit, and more particularly still to a blocking or inhibiting function or operation for preventing a trip operation in an arc fault detection system or arc fault circuit interrupter (AFCI) under selected conditions.
The electrical systems in residential, commercial and industrial applications usually include a panel board for receiving electrical power from a utility source. The power is then routed through protection devices to designated branch circuits supplying one or more loads. These overcurrent devices are typically circuit interrupters such as circuit breakers and fuses which are designed to interrupt the electrical current if the limits of the conductors supplying the loads are surpassed.
Circuit breakers are a preferred type of circuit interrupter because a resetting mechanism allows their reuse. Typically, circuit breakers interrupt an electric circuit due to a disconnect or trip condition such as a current overload or ground fault. The current overload condition results when a current exceeds the continuous rating of the breaker for a time interval determined by the trip current. A ground fault trip condition is created by an imbalance of currents flowing between a line conductor and a neutral conductor which could be caused by a leakage current or an arcing fault to ground.
Arcing faults are commonly defined as current through ionized gas between two ends of a broken conductor or at a faulty contact or connector, between two conductors supplying a load, or between a conductor and ground. However, arcing faults may not cause a conventional circuit breaker to trip. Arcing fault current levels may be reduced by branch or load impedance to a level below the trip curve settings of the circuit breaker. In addition, an arcing fault which does not contact a grounded conductor or person will not trip a ground fault protector.
There are two types of arcing faults in electrical circuits and wiring: Parallel and Series.
Parallel arcing occurs when there is an arc between two wires or wire-to-ground and the current is limited by the impedance of the voltage source, the wire, and the arc. When the fault is solidly connected and the arc voltage low, the normal breaker trips very quickly with little heating of the wire or damage at the arc point. Occasionally, however, the arc blows apart the faulted components creating a larger arc voltage and reducing the fault current below the trip curve and causing xe2x80x9cticking faults.xe2x80x9d The consequences of parallel arc damage, are usually much greater than series arcs. The average current may not be sufficient to trip a conventional breaker by heating the bimetal strip or the peak current may not be large enough to trigger the magnetic trip latch. This makes the conventional breaker reasonably effective in protecting against parallel arcing when the peak current is a few hundred amps. Unfortunately, the fault current can be limited by a circuit with too much impedance to immediately trip the thermal-magnetic breaker. Parallel arcing is generally more hazardous than series arcing. The energy released in the arc is much higher with temperatures often in excess of 10,000 Deg. F. This causes pyrolyzation or charring of the insulation, creating conductive carbon paths.
Series arcing begins with corrosion in pin-socket connections or loose connections in series with the electrical loads. The voltage drop across a poor connection begins at a few hundred millivolts and slowly heats and oxidizes or pyrolizes the surrounding materials. The voltage drop increases to a few volts at which time it becomes a xe2x80x9cglowing connectionxe2x80x9d and begins to release smoke from the surrounding polymer insulation. Series arc current is usually limited to a moderate value by the impedance of the electrical load that is connected to the circuit. The amount of power from series arc is typically far is less than in a parallel arc fault. Since the peak current is typically never greater than the design load current, series arcing is much more difficult to detect than parallel arcing. The signature of the series arc is an unusual variation of the normal load current. Series arcing is usually such that the arc current remains well below the trip curve of the breaker. Loose terminal lugs, misarranged or cross-threaded electrical plugs, broken conductor strands inside a wire are typical sources. These arcs cause load voltage drops and heating of the wire, plug pin, or terminal lug. This heating can lead to component failure and ignition sources.
There are many conditions that may cause an arcing fault. For example, corroded, worn or aged wiring, connectors, contacts or insulation, loose connections, wiring damaged by nails or staples through the insulation, and electrical stress caused by repeated overloading, lightning strikes, etc. These faults may damage the conductor insulation and cause the conductor to reach an unacceptable temperature.
Standard overcurrent devices used in circuit breakers respond to the heating effect of current in a resistive wire to xe2x80x9cthermal tripxe2x80x9d the breaker, but these do not respond to the sputtering arc currents. We propose a better approachxe2x80x94to stop the arc when it happens rather than wait for a circuit breaker to thermal trip. Until recently, such arc detection capability has not been available in circuit breakers or relays. Ground Fault Circuit Interrupters (GFCI) for personnel protection have been available in the home since the early 1970""s. Under ideal conditions, GFCI can detect phase to ground arcs as low as six milliamps, but cannot detect series arcs or improve line to neutral fault trip times.
Arc Fault detection technologies are a new and exciting innovation in circuit protection in the U.S. We have found that Arc Fault Circuit Interrupters (AFCI) can be designed to detect a series or parallel arc, as well as line to neutral arcs by xe2x80x9clisteningxe2x80x9d for the unique signatures which arcs generate. An arc fault circuit interrupter is a device intended to provide protection from the effects of arc faults by recognizing characteristics unique to arcing and by functioning to de-energize the circuit when an arc fault is detected.
Conventional circuit breakers have historically been the best available protection for wiring. Today""s design standards are based on technologies that are up to 40 years old. In circuit breakers, the protection is usually provided in two ways. Short circuit currents operate a magnetic trip latch, while overload currents operate either a bimetal trip latch or hydraulic damped magnetic plunger. The xe2x80x9cinstantaneous tripxe2x80x9d is the high current magnetic trip action found on some but not all breakers. The time to trip during an overload is determined by the time it takes to heat a bimetal to the temperature that delatches the breaker. The more current that heats the bimetal, the shorter the time it takes to trip the breaker. A hydraulic-magnetic style of breaker contains a magnetic slug sealed in fluid which moves to a trip position in response to the square of the current. These circuit interruption devices are selected by design engineers to protect the wiring from overheating or melting. During arcing faults these currents are often small, short in duration and well below the over current time protection curve designed into these breakers.
Arcing in a faulted AC circuit usually occurs sporadically in each half cycle of the voltage waveform. The complex arcing event causes sputtering arc""s that vary the current from normal load patterns. The precursor to the arc may be a high resistance connection leading to a xe2x80x9cglowing contactxe2x80x9d and then a series arc, or a carbon track leading to line-to-line or parallel arcing. In a home circuit breaker equipped with Ground Fault Circuit Interrupter (GFCI), a carbon or moisture track can be detected early if the short is to ground. With the introduction of AFCI breakers, protection of arcing shorts from line-to-line, not involving ground, can also be detected and interrupted.
In our arc fault interrupter, the additional electronic devices monitor both the line voltage and current xe2x80x9csignatures.xe2x80x9d In a normal operating circuit, common current fluctuations produce signatures which should not be mistaken for an arc. Starting currents, switching signatures and load changes (normal or xe2x80x9cgood arcxe2x80x9d events) can be digitally programmed in the AFCI as normal signatures waveforms. Deviations or changes from these xe2x80x9cnormalxe2x80x9d signatures are monitored by electronic circuits and algorithms to determine if arcing is occurring. When these arc fault signatures are recognized, the circuit is interrupted and power is removed. The speed of this detection as well as the arc magnitude can be programmable parameters at the time of manufacture. The particular signatures identified as arcs are part of the proprietary arc fault technology of Square D Company.
Commercial, UL approved AFCI circuit breakers are available. These are now in the NEC and will be required in home bedroom circuits in 2002. Since the electrical loads in residential circuits can vary widely, they should be designed to allow for almost an infinite combination of electrical loads. Their AFCI programming may be combined with GFCI as well as magnetic and thermal overload components. They may be designed to form fit and function in place of standard residential circuit breakers.
Summarizing briefly, heat, arcs or electrical ignition are often caused by loose connections, broken or shorted wires in the power distribution system. In wiring, vibration, moisture temperature extremes, improper maintenance and repair all contribute to wiring failure. This leads to arcing and may ignite combustible components. Furthermore, carbon tracking caused by heat generated by the arc can deteriorate the wire insulation, exposing the conductors and resulting in intermittent short circuits between individual wires. These inter-wire shorts can cause damage and malfunctions. Elimination or reduction of these hazards with arc fault technology should become an industry-wide priority.
It is a general object of the present invention to provide an arc fault detection system and method which reliably detects arc fault conditions which may be ignored by conventional circuit interrupters.
A related object of the invention is to provide an arc fault detection system which utilizes highly reliable electronic components, so as to be relatively simple and yet highly reliable in operation.
A more specific object is to provide an arc fault detection system in which a blocking/inhibit function is implemented to prevent the production of a trip signal under predetermined conditions.
Other and further objects and advantages of the invention will be apparent to those skilled in the art from the present specification taken with the accompanying drawings and appended claims.
In accordance with one aspect of the invention, there is provided a system for determining whether arcing is present in an electrical circuit comprising a sensor for monitoring a current waveform in an electrical circuit, an arc fault protection circuit which determines whether an arc fault is present in response to said sensor, the arc fault detection circuit including a controller which produces a trip signal in response to a determination that an arcing fault is present in said electrical circuit, and an inhibitor which prevents production of said trip signal by the controller under one or more predetermined conditions.